| Summary: | Defect structure of nominally pure lithium niobate crystals grown from a boron doped charge have been studied by Raman and optical spectroscopy, laser conoscopy, and photoinduced light scattering. An influence of boron dopant on optical uniformity, photoelectrical fields values, and band gap have been also studied by these methods in LiNbO<sub>3</sub> crystals. Despite a high concentration of boron in the charge (up to 2 mol%), content in the crystal does not exceed 10<sup>−4</sup> wt%. We have calculated that boron incorporates only into tetrahedral voids of crystal structure as a part of groups [BO<sub>3</sub>]<sup>3−</sup>, which changes O–O bonds lengths in O<sub>6</sub> octahedra. At this oxygen–metal clusters MeO<sub>6</sub> (Me: Li, Nb) change their polarizability. The clusters determine optically nonlinear and ferroelectric properties of a crystal. Chemical interactions in the system Li<sub>2</sub>O–Nb<sub>2</sub>O<sub>5</sub>–B<sub>2</sub>O<sub>3</sub> have been considered. Boron, being an active element, structures lithium niobate melt, which significantly influences defect structure and physical properties of a crystal grown from such a melt. At the same time, amount of defects Nb<sub>Li</sub> and concentration of OH groups in LiNbO<sub>3</sub>:B is close to that in stoichiometric crystals; photorefractive effect, optical, and compositional uniformity on the contrary is higher.
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